1. Field of the Invention
This invention relates to an electric linear actuator. A linear actuator is utilized, for example, for driving a flow rate control valve or the like disposed in a hydraulic circuit.
2. Related Background Art
In a hydraulic circuit provided, for example, in an automotive vehicle, an electric linear actuator is preferably employed in order to drive a flow rate control valve based on an instruction from a controller. Generally, linear actuator constituted by a combination of a solenoid and a compression spring has been used. This type of linear actuator is constructed such that a displacement member such as a rod is urged by the compression spring in one of the axial directions, and when this displacement member need is to be displaced, the solenoid is energized to displace the displacement member in the axial direction against the urging force of the compression spring. The displacement member is displaced and stopped at a position where the urging force of the compression spring balances with a force of the solenoid. A displacement amount of the displacement member is adjusted by controlling the amount of current to the solenoid and changing the magnitude of force which the solenoid exerts on the displacement member.
An electric linear actuator utilizing a solenoid as mentioned above requires a large current to be supplied to the solenoid when the displacement member is to be displaced against the urging force of the compression spring. However, a large current unfavorably causes an excessive load on a battery. Also, the structure for urging a displacement member by a compression spring as mentioned above presents large hysteresis, whereby even if an identical amount of current is supplied to the solenoid, a displacement amount of the displacement member can vary widely. For this reason, it is quite difficult to finely adjust an opening angle of a flow rate control valve by a linear actuator.
To solve the above problem, the present assignee has proposed an electric linear actuator as shown in FIGS. 11-12 (Japanese Patent Application No. 3-201142). Referring to FIG. 11, this electric linear actuator comprises an output shaft 124 made displaceable only in the axial direction by a support 122 formed in a housing 120; a cylinder 126 arranged concentrically with this output shaft 124 for rotation about it; a torsion coil spring 130 arranged between the cylinder 126 and a casing 128 for applying to the cylinder 126 an urging force in the circumferential direction; a feed screw 135 arranged between the outer peripheral surface of the output shaft 124 and the inner peripheral surface of the cylinder 126 for displacing the output shaft 124 in the axial direction with the rotation of the cylinder 126; and an electric motor 132 arranged inside the casing 128 for rotating the cylinder 126, when energized, against the urging force of the torsion coil spring 130. The feed screw 135 is constituted by a spiral groove 138 formed in the outer peripheral surface of the output shaft 124, another spiral groove 140 formed in the inner peripheral surface of the cylinder 126, and a multiplicity of balls 142 accommodated between both grooves. An end of the output shaft 124 is inserted into a guiding portion 121 of the housing 120 which guides the output shaft 124 so as to block its rotation and allow displacement only in the axial direction.
In the above-mentioned electric linear actuator, when the electric motor 132 is not energized, the cylinder 126 is subjected to the rotational fore of the torsion coilspring 130. This force acts, by way of the feed screw 135, to bring the output shaft to one end of its range of movement in th axial direction. On the other hand, when the electric motor 132 is energized, the cylinder 126 is rotated until a rotating torque of the electric motor 132 determined in accordance with an amount of current balances with the urging force of the torsion coilspring 130, and the cylinder 126 is stopped in the balanced state. With the rotation of the cylinder 126, the output shaft 124 is displaced by the feed screw 135 toward the other end of its range of movement in the axial direction.
The above-mentioned electric linear actuator solves the problem inherent in conventional electric linear actuators utilizing a solenoid by virtue of the feed screw which smoothly operates to convert the moving direction of the output shaft. However, the following problems are still left unsolved.
Specifically, since the feed screw 135 for displacing the output shaft 124 in the axial direction with the rotation of the cylinder 126 is realized by a ball and thread mechanism constituted by the spiral grooves 138, 140 and balls 142, it requires a returning tube for the balls, which have moved to ends of the spiral grooves 138, 140, to be returned to the other ends of the spiral grooves 38, 140. Consequently, the feed screw 135 becomes complicated and large in size, thereby presenting difficulties in designing an electric linear actuator to be installed in a narrow area.